[Brightcolours]

Canon RF 100-500mm f/4-7.1L IS USM £2,899

Are they nuts? Compare this to the price of the Sony 200-600mm f/5.6-6.3 G OSS ... 1699 GBP ...

£1,800, actually.


 Can be found at under €1,700 ......

   F7.1 ????  ..... is this some kind of slow insidious decline in aperture size from Canon that is there to soft sell us with time?? ...... or is there someone at Canon who thinks that wide angle to normal FL lenses ideally need to be F1.2 (and $3,000)  in order to keep shutter speeds at around 1/4000 sec for landscape work ?? ......

....... while BIF images are best taken at small apertures in order to be able to blur the action to portray artistic  movement ?? ...... which is what you'll get from slower shutter speeds

...... Nikon didn't want to drop below F5.6 ....... with a FL from 200-500mm ..... and kept the price down to $1,400 ....... (now btw way you can buy it for €1,035 )

  If you want 600mm the Sony gets the cigar here at F6.3

  Sony and Nikon are the clear winners here !!

The MSRP is 1800, not 1700. 
It is a pricy lens indeed, that Canon 100-500mm.  Its pricing is in line with the EF 100-400mm L IS USM II, not with the Nikkor 200-500mm and Sony 150-600mm.
Canon is, at the moment, making lighter and smaller lenses for mirrorless, believing that smaller is the big plus for mirrorless in the 1st place. So, the extending 70-200mm f2.8 L, the extending DO 600mm f11 and Do 800mm f11, and this 100-500mm f4.5-7.1. 
The Canon RF 100-500mm weighs 1370 grams + 160 grams tripod ring = 1530 grams
The Nikkor 200-500mm weighs 2300 grams
The Sony 200-600mm weighs 2410 grams
The difference is very clear, there. Whether you prefer a bit faster max. aperture or less weight, that is a personal choice. Canon has chosen a lower weight. 
If you would chose a Canon R body and would like to spend less, or have the aperture range the Sony offers, you can of course choose the Tamron 150-600mm G2. That lens weighs 2110 grams in total.
For hiking, that Canon with its low weight is attractive. And the price, yes, that is high. You can offset that with the low price of the R6 of course ;-)
And, of course, a Panasonic MFT body with 100-300 f4-5.6 (200-600mm f8-11 FF equivalent) is even lighter,. Choices.

My personal impossible combo would be the Canon EOS R6, with Nikkor AF-S 200-500mm f5.6 (and Canon 35mm f2, 85mm f2 and 50mm f1.2 and 18-45mm).